1. Field of the Invention
The present invention relates to signal line changeover circuits, and more particularly, to signal line changeover circuits for use in integrated circuits.
2. Description of the Prior Art
For example, among color signal processing circuits of a video cassette recorder (VCR) consisting of the integrated circuit (IC), an automatic color control (ACC) circuit has, as shown in FIG. 1, a portion common to both a recording system and a playback system. When a signal is inputted to the common portion, a changeover of the input signal is performed by a selection switch (recording/playback changeover switch).
In the Fig. 1, solid lines show the recording system, while alternate long and short dash lines show the playback system. Blocks 3, 6, and 8 are common to the recording and playback systems. In the recording system, a color signal inputted into an input terminal 1 passes through a band pass filter 2 with a center frequency of 3.58 MHz, and thereafter, is provided to a gain variable amplifier 3 for ACC. An output of the gain variable amplifier 3 is decreased in frequency to 629 kHz by a balanced modulation circuit 4, and thereafter, is directed to an output terminal 60 by way of a low pass filter 5. The output of the gain variable amplifier 3 is also provided to a color burst level detection circuit 8 through a selection switch 7 in the switch block 6. An output of the color burst level detection circuit 8 is supplied to the gain variable amplifier 3 so as to control a gain of the gain variable amplifier 3.
In the playback system, a playback color signal inputted into an input terminal 9 passes through a band pass filter 10 with a center frequency of 629 kHz, and thereafter, is provided to the gain variable amplifier 3. In this case, an output of the gain variable amplifier 3 is converted into a signal which has a carrier of 3.58 MHz by the succeeding balanced modulation circuit 4. Then, after passing through the band pass filter 2 of 3.58 MHz and a comb filter 11, it is amplified by an amplifier 12, and thereafter, is directed to an output terminal 13. The color signal directed to the output terminal 13 is further directed to a Y/C mixer (not shown) connected to the output terminal 13 to be combined with a luminance signal.
In FIG. 1, the selection switch 7 in the switch block 6 is set to a contact R side during a recording operation so that the output of the gain variable amplifier 3 is directed to the detection circuit 8. During a playback operation, the switch 7 is set to a contact P side so that an output of the amplifier 12 is directed to the detection circuit 8.
FIG. 2 shows a main portion of the circuit shown in FIG. 1, with which a problem will be described. In FIG. 2, during a recording operation when the selection switch 7 is set to the contact R, a color signal from the gain variable amplifier 3 is supplied to the color burst level detection circuit 8 through the switch 7, while the output of the amplifier 12 is blocked by the switch 7 and is not coupled to the detection circuit 8. Moreover, during a playback operation when the selection switch 7 is set to the contact R, a color signal from the amplifier 12 is supplied to the detection circuit 8 through the switch 7, while the output of the gain variable amplifier 3 is blocked by the switch 7 and is not coupled to the detection circuit 8.
Since an output signal line 14 of the gain variable amplifier 3 and an output signal line 15 of the amplifier 12 run long to the switch block 6 in the IC, there arises problems that an un-used output signal line influences the other signal line and that it influences an adjoining third circuit on the way to the switch block 6. The problems are conspicuous particularly with respect to a circuit where signal lines are densely arranged such as a large scale integrated circuit.
To solve the above-described problems, a method may be used where a switch is provided to each signal block (in FIG. 2, the gain variable amplifier 3 and the amplifier 12) in order that no signal is outputted to an output signal line when the signal of the block is unnecessary. In that case, however, not only the size of each block (3, 12) should be increased but also another control line should be provided for controlling the switch provided to each block, whereby the structure of the IC becomes more complicated because of the increased number of lines as well as a degree of integration decreases.